1. Field of the Invention
The present invention relates to a process for converting vinyl alcohol polymers. More particularly, the invention involves a process whereby the hydroxyl groups of vinyl alcohol polymers are oxidatively converted to carbonyl(keto) and oxycarbonyl(ester) moieties.
2. Background of the Invention
Vinyl alcohol polymers, including homopolymers and copolymers, are known. They are typically obtained by the partial or complete hydrolysis of a poly(vinyl ester), most commonly poly(vinyl acetate) or a vinyl ester copolymer such as an ethylene-vinyl acetate copolymer. While numerous hydrolysis procedures can be used to obtain vinyl alcohol polymers, the most commonly used method employs a stoichiometric amount of base in water, e.g. aqueous NaOH. Ester interchange reactions which are carried out in the presence of an alcohol employing a catalytic amount of a base are also widely used to produce vinyl alcohol polymers. A variety of vinyl alcohol polymers including fully hydrolyzed products, typically having less than 1.5 mole percent residual acetate functionality, and partially hydrolyzed products, typically containing as much as 20 mole percent or more residual acetate groups, are commercially available.
The reactivity of the hydroxyl groups of vinyl alcohol polymers and their ability to undergo reactions typical of compounds containing secondary hydroxyl groups are well known and widely reported in the literature. For example, the hydroxyl group can be converted to borate, nitrate, phosphate and sulfate groups; they will react with carboxylic acids, carboxylic acid anhydrides and carboxylic acid chlorides, to form esters; they will react with alkylene oxides to form ethers; they will react with aldehydes to produce acetals; they will react with ketones to produce ketals, etc. For a discussion of these and other reactions of poly(vinyl alcohols) reference may be had to Encyclopedia of Polymer Science and Technology, Vol. 14, Interscience, a division of John Wiley & Sons, New York, 1971, pp. 169-174. In the same reference it is reported that poly(vinyl alcohols) undergo crosslinking or degradation when subjected to oxidation by air (under alkaline conditions), t-butyl hypochlorite, uranyl acetate, and chromic acid. Hydrogen peroxide is reported to oxidatively degrade poly(vinyl alcohol) under acidic conditions.
British Patent Specification 1066730 discloses a process for oxidizing poly(vinyl alcohols) to produce polyenols of enhanced thermal stability consisting predominantly of recurring units of the structure ##STR1##
The process entails oxidizing a poly(vinyl alcohol) in an aqueous alkaline medium using a mixture of cupric oxide or cupric hydroxide and a noble metal or noble metal oxide or hydroxide. While the reference does not mention the presence of keto groups in the polymer, the existence of such groups could reasonably be expected in view of the well recognized ability of enol groups to rearrange.
Polymeric hydroxy keto esters have been obtained by the process of U.S. Pat. No. 2,557,256. They are produced by reacting carbon monoxide, a polymerizable organic compound containing ethylenic unsaturation and an alcohol or an alkyl formate. These polymers have repeating keto units of the formula ##STR2## and repeating units of the formula ##STR3## combined, where A is the residue from the polymerizable organic compound (C.sub.2 H.sub.4 in the case of ethylene) and R is the non-hydroxyl portion of the alcohol or alkyl group of the alkyl formate.
U.S. Pat. No. 3,976,621 discloses a process for rendering ethylene-vinyl alcohol copolymers photodegradable by reacting the copolymer with .alpha.-diazoacetophenone in the presence of an organic solvent and a catalyst consisting of a Lewis acid. The copolymers have .alpha.-ketoether groups pendant to the polymer chain.
U.S. Pat. No. 4,929,711 discloses a process for converting polyketones, such as are obtained by the polymerization of ethylene and carbon monoxide, to polyesters utilizing an organic peroxyacid oxidizing agent. A similar process for converting polyketones containing pendant functional groups to polyester is disclosed in U.S. Pat No. 4,957,997. U.S. Pat. No. 5,064,932 discloses poly(keto-esters). produced by the foregoing processes and comprised of carbonyl units, oxycarbonyl units and linking units derived from olefinic monomers.
It would be highly advantageous if a process were available whereby hydroxyl groups in vinyl alcohol polymers could be readily oxidized to carbonyl and oxycarbonyl groups. It would be even more advantageous if useful polymeric products were produced and if the oxidation did not substantially affect other functionality that might be present in the case of copolymers. It would be still more advantageous if the degree of oxidation could be controlled so that substantially all or only a portion of the hydroxyl functionality would be oxidized to carbonyl as well as oxycarbonyl moieties and if, as a result, the products were rendered both photodegradable and biodegradable. These and other advantages are realized by the process of the present invention and will be described in more detail to follow.